Shielding Analysis of a Preclinical Bremsstrahlung X-ray FLASH Radiotherapy System within a Clinical Radiation Therapy Vault.

ABSTRACT: A preclinical radiotherapy system producing FLASH dose rates with 12 MV bremsstrahlung x rays is being developed at Stanford University and SLAC National Accelerator Laboratory. Because of the high expected workload of 6,800 Gy w -1 at the isocenter, an efficient shielding methodology is needed to protect operators and the public while the preclinical system is operated in a radiation therapy vault designed for 6 MV x rays. In this study, an analysis is performed to assess the shielding of the local treatment head and radiation vault using the Monte Carlo code FLUKA and the empirical methodology given in the National Council on Radiation Protection and Measurements Report 151. Two different treatment head shielding designs were created to compare single-layer and multilayer shielding methodologies using high-Z and low-Z materials. The multilayered shielding methodology produced designs with a 17% reduction in neutron fluence leaking from the treatment head compared to the single layered design of the same size, resulting in a decreased effective dose to operators and the public. The conservative assumptions used in the empirical methods can lead to over-shielding when treatment heads use polyethylene or multilayered shielding. High-Z/Low-Z multilayered shielding optimized via Monte Carlo is shown to be effective in the case of treatment head shielding and provide more effective shielding design for external beam radiotherapy systems that use 12 MV bremsstrahlung photons. Modifications to empirical methods used in the assessment of MV radiotherapy systems may be warranted to capture the effects of polyethylene in treatment head shielding.

Quantification of Activity Lost to Delivery-System Residual and Decay in Yttrium-90 Radioembolization.

PURPOSE: To measure the decay activity loss and delivery system residual activity loss of yttrium-90 (Y90) radioembolization treatments across resin and glass microsphere activities. MATERIALS AND METHODS: For Y90 administrations between December 2009 and June 2017 at the study institution, the prescribed activity, prepared activity, and delivered activity were recorded. Six hundred sixty-two administrations were reviewed-345 glass (0.21-8.52 GBq) and 317 resin (0.18-3.28 GBq). Twenty-five patients (all resin) were excluded for arterial stasis or catheter clogging. The percentage and actual losses of activity lost to decay and to delivery system residual were calculated for glass and resin microspheres. RESULTS: The median time between activity premeasurement and administration was 2.20 hours, resulting in a median activity lost to decay of 0.030 GBq or 2.35%, with no significant difference observed between glass and resin despite differences in preparation (P = .0697). Resin showed significantly higher activity lost to delivery system residual than glass (0.039 GBq vs 0.010 GBq, 3.01% vs 0.61%, P < .001). The percent activity lost to residual varied with activity prepared, with a maximum of 20.1% and 16.2% for the smallest activities of resin and glass, respectively. CONCLUSIONS: Residual activity loss differs between glass and resin microspheres. For resin microspheres in particular, percent residual activity loss increases with lower prepared activities. Protocols for activity calculation and preparation, patient dosimetry, and regulatory compliance must take these losses into consideration prospectively.